Rafael Nadal admits 'I had doubts' before U.S. Open triumph

(CNN) -- Crushing a string of opponents before dispatching Novak Djokovic in four sets in the U.S. Open final, Rafael Nadal was back to his ruthless best, with not a hint of the injury problems which have threatened to blight his career, even during a year which has seen him claim two grand slams.

But the Spaniard has admitted to CNN that he had real fears that as he made yet another comeback he would fall short of his own incredible standards.

"Sure I had doubts, everyone has doubts," said the 27-year-old Mallorcan.

"When you are injured you doubt if you going to be able to come back and play at 100%."

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Closing fast on Djokovic's No.1 spot, Nadal has spent two lengthy spells on the sidelines through knee injuries, both times making a strong comeback.

Read: Nadal crashes to stunning Wimbledon defeat

Returning to the ATP Tour in February, Nadal clicked straight into gear, culminating in his eighth victory in the French Open.

But come Wimbledon his battered body could take no further punishment and Nadal was a shadow of his normal self as he crashed out in the first round to unheralded Steve Darcis, clearly troubled by knee problems.

The doubters were out in force again and with Nadal sidelined yet again, Djokovic and Wimbledon champion Andy Murray were predicted to share the grand slams between them in the foreseeable future.

They were reckoning without Nadal's incredible determination and joining the circuit again for the pre U.S. Open hard court series, the Mallorcan looked as if he had never been away, claiming titles in Montreal and Cincinnati before his Flushing Meadows triumph.

The mixture of joy and relief were there for all to see as Nadal savored every moment on the Arthur Ashe Stadium Court in claiming his 13th career grand slam.

"When you are coming back after low moments victories are all the more special and more emotional," Nadal admitted.

Read: Nadal wins 13th grand slam crown

"I enjoy it now because in a few years I won't have the chance to play like I did in the U.S. Open final."

The manner of his victory suggested that Nadal could easily outstrip all-time grand slam record holder Roger Federer, who has 17 in his trophy room.

But always at the back of Nadal's mind is the possibility his knee tendon problem might flare up again so he is taking nothing for granted.

"We need our knees, without our knees it is impossible to play well, sometimes I still have pain, but the most important thing is that at the moment I am able to play without limitations," said the newly crowned U.S. Open champion.

Nadal has been a force on the circuit for approaching a decade since bursting to prominence as a precocious teenager, particularly on his favorite clay surface.

Read: Nadal back to No.2 with Cincinnati triumph

His all action game was an immediate crowd pleaser but he admits that he takes a more measured approach these days.

"When you are a kid you are able to play with a different style, crazy jumping and all that," added Nadal.

"That moment has passed but my love and passion for the game is still the same."

All that spells bad news for those battling him at the elite end of the men's game with plenty to play for still in 2013, including the ATP World Tour Finals in London, just about the only title which has eluded him.